Process for the production of propylene copolymers

ABSTRACT

Polymers of propylene with one or two olefin comonomers are produced through the use of a poymerisation catalyst supported on a carrier produced by spray crystallisation and having a maximum dimension of μm. Copolymers of superior properties are obtained, and the production process is de-snagged.

The present invention relates to the production of propylene polymers,in particular copolymers of propylene with one or two olefin comonomers.The copolymers are suitable for applications where good heat sealabilityand softness are required.

Polyolefins used in film applications as sealing layers should have lowmelting temperatures for good heat sealing performance. In polypropylenefilms, random copolymers having a relatively high content of comonomers,usually ethylene, are commonly used as such a sealing component. A highcomonomer content is required for obtaining low melting temperatures.There are, however, problems associated with the process of productionof random copolymers and terpolymers (i.e. propylene polymers with twocomonomers), and with the end-properties of the materials themselves,because of their low melting temperature, in-process stickiness, andpolymer solubility in the polymerisation medium. Further, catalystactivity increases significantly during copolymerisation, particularlyin terpolymerisation, leading to undesirable growth in polymer particlesize.

In comparison to homopolymer production, when a terpolymer or highcomonomer content copolymer having a low melting temperature is to beproduced in a slurry reactor, the solubles content of the polymer ishigh. Further, the comonomers used in the polymerisation, such asethylene and/or butylene, cause swelling of the polymers in thepolymerisation medium of the slurry reactor, resulting in excessivelylarge and soft polymer particles. There is also a higher demand forexternal heat if it is desired to evaporate unreacted monomer byflashing after the slurry reactor, which may cause surface melting ofpolymer particles. If pressure in the flash vessel is reduced too muchto improve monomer evaporation, flashing takes place too fast and themorphology of the particles is destroyed, causing problems downstream.

Because of the above-mentioned increased catalyst activity, productpolymer particles can grow too rapidly and to too great a size, causingpipe plugging and product inhomogeneity and attrition. Moreover if agas-phase reactor is employed, fluidisation efficiency and continuoustake-out may be compromised. Further downstream problems may also arisein post-polymerisation procedures and units, such as catalystdeactivation, steaming and drying.

Polymerisation catalyst particle size is principally determined by theparticle size of the carrier upon which catalytically active matter issupported. Such carriers can be produced by various processes such asspray crystallisation, spray-drying or emulsion processes. When, as iscommon, magnesium dichloride is used as carrier, the process is referredto as spray crystallisation. Typically a melt of MgCl₂ in a suitablequantity of a lower alcohol such as methanol is fed to a spray dryeroperating at 150-200° C., whereupon the alcohol is flashed off and a drymagnesium chloride crystallite powder of quite broad particle sizedistribution is obtained.

The particle size extremes of the carrier powder range from 1 μm to 1.4mm or even higher, the mass distribution between these limits followingthe usual bell curve. The powder is separated, according to standardpractice, by screening to remove large particles, agglomerates and smallparticles. The intervening fraction is generally employed for catalystmanufacture, the large particles and agglomerates are usually recycledto melt preparation, and the small particles are discarded as beingunusable.

Such a procedure is disclosed in WO-A-87/7620, in which a <74 μMfraction of small particles is separated by screening from a 10-300 μmspray-crystallised product. According to U.S. Pat. No. 5,100,849 bothsmall and large particles are removed from the spray-crystallisedproduct to yield a 45-212 μm intervening fraction as carrier product.EP-A-700,936 stresses the desirability of good catalyst shape andrelatively large particle diameter, its carrier being the interveningfraction remaining after screening to remove <65 and >120 μm particles.

It has now been discovered that if, instead of the intervening material,the smallest-crystallite fraction is employed as carrier in catalystmanufacture, the aforesaid problems in the manufacture of propylenecopolymers and terpolymers can be effectively avoided.

According to the present invention, therefore, a process for producing acopolymer of propylene with ethylene and/or a C₄ to C₈ α-olefincomprises contacting propylene and said ethylene and/or olefin underpolymerisation conditions with an olefin polymerisation catalyst whichincludes a carrier, the carrier particles included in said catalystbeing substantially free of particles having a minimum dimension greaterthan 55 μm. In the preferred practice of the invention, the copolymer isa propylene/ethylene/C₄-C₈ α-olefin terpolymer.

In a preferred embodiment the carrier is produced by spraycrystallisation of a melt of a carrier precursor to yield aspray-crystallised product containing particles up to at least 500 μm insize.

Thus, following standard procedure, the catalyst carrier may, after itsformation, be separated into a plurality of fractions, including onehaving a maximum particle dimension of 55 μm, by screening. Thepreferred carrier is magnesium dichloride, and a suitable melt for feedto the spray-crystallizer is represented by the formula:MgCl₂.nROH.mEDin which R is C₁-C₆ alkyl, ED is an electron donor, m=0-1 and n=1-6. Theelectron donor, ED, is preferably an aromatic carboxylic acid ester,particularly a dialkyl phthalate or maleate, most preferably di-2-ethylhexyl phthalate.

It is not necessary that all ingredients of the final copolymer bepresent at the start of polymerisation. Production of the copolymer orterpolymer may be commenced by contacting propylene alone with thecatalyst. Production of the terpolymer may be commenced by contactingpropylene and ethylene and/or said α-olefin with the catalyst. Thecontacting may be performed in at least one slurry reactor and/or atleast one gas phase reactor. In a preferred embodiment of the inventionthe contacting is performed in at least one slurry reactor and at leastone gas phase reactor, the gas phase reactor or reactors beingadvantageously downstream of the slurry reactor or reactors.

When the copolymer is a terpolymer it preferably comprises 0.1 to 3 wt %of units derived from ethylene and 1 to 15 wt % of units derived fromC₄-C₈ α-olefin, and the α-olefin of choice is butene.

The carrier of the polymerisation catalyst preferably has an averageparticle size of 15 to 45 μm. The olefin polymerisation catalyst itselfis suitably a stereospecific Ziegler-Natta catalyst system comprising acomponent containing Ti, Cl and Mg as essential components; acocatalyst; and an external electron donor. The cocatalyst is preferablyan organoaluminium compound, such as triethylaluminium, and thepreferred electron donors are silane-based donors. Generally, the silanebased donors have the formula (I)R_(n)R′_(m)Si(R″O)₄-n-m  (I)wherein

-   R and R′ can be the same or different and stand for linear, branched    or cyclic aliphatic or aromatic group;-   R″ is methyl or ethyl;-   n is an integer 0 to 3;-   m is n integer 0 to 3; and-   n+m is 1 to 3.

The aliphatic groups in the meanings of R and R′ can be saturated orunsaturated.

Preferably, R and R′ are linear C, to C₁-2 hydrocarbons which includemethyl, ethyl, propyl, butyl, octyl and decanyl. As examples of suitablesaturated branched C₁-8 alkyl groups, the following can be mentioned:isopropyl, isobutyl, isopentyl, tert.-butyl, tert.-amyl and neopentyl.Cyclic aliphatic groups containing 4 to 8 carbon atoms comprise, e.g.cyclopentyl, cyclohexyl, methylcyclopentyl and cycloheptyl.

Alternatively, the olefin polymerisation catalyst may be a single-sitecatalyst comprising a single-site/activator reaction product impregnatedin the pores of the carrier.

The invention further embraces the use of spray-crystallised MgCl₂having an average particle size below 55 μm as carrier for apolymerisation catalyst in the production of propylene copolymers orterpolymers.

It is particularly preferred to produce the material in a combination ofone or more slurry polymerisation reactor(s) and one or more gas phasereactor(s). Thereby, the comonomer conversion can be increased, andcomonomer distribution can be optimised and tailored. However it ispossible to employ a slurry-slurry (preferably loop-loop) system; or twogas phase reactors; or any combination thereof. It is known (theso-called “replication effect”) that the physical structure of thepolymerisation catalyst carrier is reproduced in the overall catalystcomposition, and eventually in the particulate polymer product itself.In the process according to the invention, therefore, the use of thesmall particle size catalyst leads to the formation of a correspondinglysmall particle size product. This is particularly advantageous becauseof the concomitant reduction in production of large particles which,through breakdown, are normally the principal source of undesiredproduct fines.

A favoured procedure for terpolymer production thus comprises:

-   -   a) feeding into a slurry reactor a mixture containing 50 to 85        wt % of propylene, 0.1 to 10 wt % of ethylene, 3 to 40 wt % of        another α-olefin, a catalyst system capable of achieving olefin        polymerisation at said temperature conditions, and optionally        hydrogen,    -   b) polymerising said reaction mixture at a temperature of less        than 70° C. for a sufficient time to obtain a propylene        terpolymer amounting to 50 to 99.5 wt % of the end product,    -   c) transferring said reaction mixture into a gas phase reactor        operating at a pressure higher than 5 bar, preferably higher        than 10 bar, and optionally adding 0 to 10 wt % of ethylene, 0        to 10 wt % of another α-olefin, 0 to 40 wt % of propylene, of        said transferred mixture, and optionally hydrogen, and    -   d) continuing polymerisation in said gas phase reactor for        obtaining a propylene terpolymer amounting to 0.5 to 50 wt % of        the end product,

Preferably a bulk loop reactor is used as slurry reactor, and the slurryphase may be carried out in two slurry reactors, preferably but notnecessarily in two loop reactors. When continuing the polymerisation ina gas phase reactor or reactors, the comonomer distribution can beeasily controlled and comonomer content can be increased further. Thus,the final polymer properties can be tailored by adjusting comonomerratios in different reactors.

In a typical terpolymer production, a polymerisation catalyst system andmonomer mixture is fed into a slurry reactor. Propylene acts as amonomer and a diluent in the reaction mixture. The C₄-C₈ α-olefin can be1-butene, 1-pentene, 4-methyl-1-pentene, 1-hexene, 1-heptene or1-octene.

In the first slurry step a terpolymer is produced, in which the contentof ethylene monomer is in the range of 1 to 4 wt %, preferably less than3 wt %. The C₄-C₈ olefin content in the product will be in the range of5 to 12 wt %. Hydrogen is added, when desired. The temperature in theslurry reactor is preferably below 70° C.

In the gas phase reactor, 0.5 to 50 wt %, preferably 0.5 to 30 wt % ofthe final end product is formed. The polymerisation can be carried outat a temperature of 60 to 90° C. and at a pressure higher than 5 bar,preferably higher than 10 bar. Propylene and other monomers can beadded, but not necessarily, into the gas phase reactor. Hydrogen can beadded also into the gas phase reactor, when desired.

Terpolymers of propylene are obtained in which the ethylene content isgenerally 0.1 to 10 wt % and the content of other α-olefins is 1 to 25wt %. These products have low melting point and thus they can be usedfor applications such as heat-sealable sheets and films, when softnessis required. Products having low stiffness can be used in applicationslike moulded sheets, lids, bottles and fibres. It is preferred toproduce terpolymers containing less than 3 wt % units (e.g. 0.3 to 3 wt%) derived from ethylene and less than 15 wt % (e.g. 1 to 15 wt %) ofthe other units of α-olefins.

The catalyst used in such an operation, whether directed to productionof terpolymer or of a propylene/C₄-C₈ α-olefin copolymer, may be of theZiegler-Natta or of the single-site kind, supported on the prescribedcarrier.

Ziegler-Natta catalysts typically comprise the reaction product of a GpIVB metal halide with a magnesium/electron donor complex. They areusually synthesised by reacting a dialkyl magnesium with a polyhydricalcohol and with a carboxylic acid dihalide to produce the complex; andreacting the so-produced complex with a titanium tetrahalide to producea solution of the desired Ti/Mg/electron-donor catalyst component. Thiscomponent is then recovered from its solution, usually by precipitationthrough the use of aliphatic and/or aromatic hydrocarbon precipitatingagents, and subjected to conventional washing and drying. Alternativemeans of recovering it include simple evaporative removal of solvent;and its isolation as the dispersed phase of an emulsion, followed bysolidification (suitably by heating) and recovery by filtration ordecantation.

The carrier may be introduced into this sequence of operations at anystage, provided that efficient uptake of the final catalyst componentwill result. Preferably it is added to the solution of Ti/Mg/electrondonor component, or to either of the reagents (complex or titaniumhalide) which are combined to produce it. However it may be added tothat component after its recovery from its solution. When that componentconstitutes the dispersed phase of an emulsion, the emulsion ispreferably caused to form in the presence of the carrier, so that thecomponent forms in situ in the carrier's interstices.

It is possible to introduce the carrier into the synthesis at an earlierstage, for instance during or immediately after the formation of themagnesium/electron donor complex, provided that the nature of thematerials employed permits the efficient progress in the carrier'sinterstices of the subsequent reactions which are to occur.

The following examples illustrate the present invention. In them:

Melt Flow rates were measured with a load of 2.16 kg and at 230° C.according to ISO 1133.

Comonomer contents (ethylene and butene) were measured with Fouriertransform infrared spectroscopy (FTIR) calibrated with NMR.

Determination of xylene soluble fraction (XS) 2.0 g of polymer isdissolved in 250 ml p-xylene at 135° C. under agitation. After 30±2minutes the solution is allowed to cool for 15 minutes at ambienttemperature and then allowed to settle for 30 minutes at 25+0.5° C. Thesolution is filtered with filter paper into two 100 ml flasks.

The solution from the first 100 ml vessel is evaporated in nitrogen flowand the residue is dried under vacuum at 90° C. until constant weight isreached.

The xylene soluble fraction is calculated using the following equation:XS%=(100×m ₁ ×v ₀)/(m ₀ ×v ₁),wherein

-   -   m₀=initial polymer amount (g)    -   m₁=weight of residue (g)    -   v₀=initial volume (ml)    -   v₁=volume of analysed sample (ml)

EXAMPLE

Carrier Preparation:

An MgCl₂(C₂H₅OH)_(n) melt (at 115° C.) was top-fed into a spraycrystallisation chamber through an atomiser wheel to form droplets. Aninert gas, nitrogen, was fed in two parts: from the top and from themiddle part of the chamber, at respective temperatures 64° C. and −21°C. Solidified carrier particles fell to the bottom of the chamber on a53 μm screen. The retained material was discarded, and the materialpassing the screen employed as carrier in catalyst synthesis.

Catalyst Synthesis:

A highly active polypropylene polymerisation catalyst of Ziegler-Nattatype was prepared in the following manner. 1100 kg of TiCl₄ was fed to areactor and cooled to −20° C. 77 kg of the carrier was suspended in 223kg aliphatic hydrocarbon solvent (bp 90°-110° C.) and cooled beforeadding it to the cold TiCl₄. Controlled heating to 130° C. wasperformed. During heating di-2-ethyl hexyl phtalate (DOP) was added, andtransesterification (DOP→DEP, diethyl phtalate) effected by keeping themixture for 30 minutes at 130° C. The solid was separated by filtration.The procedure was repeated 3 times, each repeat adding the TiCl₄ to thefiltered solid recovered from the previous titanation. The catalyst,resulting from the third repeat, was washed 4 times with the aliphatichydrocarbon solvent and dried in vacuum to free flowing powder.

Table 1 and FIG. 1 show the particle distribution size of the carrierand of the catalyst as produced above.

Polymerisation:

A combination of a loop reactor and a gas phase reactor was used toproduce propylene terpolymers for films. Propylene, ethylene, butene andhydrogen were fed into the loop reactor. The polymer slurry produced inthe loop reactor, along with unreacted monomers and hydrogen, was passeddirectly into the gas phase reactor and polymerisation was completedtherein.

The external electron donor used was dicyclopentyl dimethoxy silane andthe activator (cocatalyst) was triethylaluminium. TABLE 1 Sample CarrierCatalyst Mean 26 17 D10 7 4 D50 33 26 D90 46 39

The polymerisation conditions and product composition are summarised inTable 3.

COMPARATIVE EXAMPLE

Example 1 was repeated, except that the carrier employed for catalystsynthesis was that passed by a 125 μm screen and retained by a 53 μmscreen.

Table 2 and FIG. 2 show the particle size distribution of the carrierand of the titanized catalyst product obtained from it. TABLE 2 SampleCarrier Catalyst Mean 75 49 D10 49 23 D50 87 63 D90 124 92

As shown in Table 3 (below), the polymerisation conditions and productcomposition were essentially the same in the Comparative Example as inthe Example of the invention. TABLE 3 Comparative Example Example Al:DOP5 5 Al:Ti (wt ratio) 300 250 Loop Temp ° C. 63 64 Productivity in loop90% 95% GPR temp 75 77 GPR pressure 16 16 GPR productivity 10% 5% Finalpolymer Ethylene wt % 1.5 1.5 1-butene wt % 6.7 6.4 XS wt % 6.5 5.6 MFRof pellets 6.6 6.0

However as shown in Table 4 (below) there were major difference in therespective polymer products' particle size distribution (screeningmethod). TABLE 4 Polymer Polymer Screen (mm) Ex (wt %) Comp Ex (wt %)4.00 0.1 0.3 3.35 0.2 0.5 2.36 0.5 11.6 1.70 14.7 32.6 1.00 65.9 39.30.50 17.1 11.9 0.15 1.3 3.3 0.06 0.0 0.2 0 (pan) 0.2 0.3

It will be observed that the polymerisation according to the inventionresulted in a product heavily (two-thirds) concentrated about the idealparticle size of 1 mm, and essentially free of >2.3 mm matter, with only15.5% of particles >1.7 mm. By contrast the comparative product wasdiffusely spread over the size range 0.5-2.36 mm, with a significantquantity of >2.3 mm particles (12.4%) present and 45% over 1.7 mm.Additionally the amount of fine particles (size range <0.5 mm) is 1.5 wt% according to the invention, versus 3.8 wt % in the comparativeexample, indicating breakage of particles. It is surprising that thereplication effect manifests itself so much more strongly in theinventive process than in the comparative.

Moreover, the production according to the Comparative Example showedcritical process disturbances. The pipe between loop and GPR was fouled,and gradually plugged. The catalyst deactivation and dryer (afterpolymerisation reactors) showed pressure disturbances leading toproduction shutdown. The polymer had a strong smell of 1-butene.

In the inventive Example the production went smoothly, without anydisturbances. When the catalyst was changed from that of the ComparativeExample to that of the inventive Example, the fluidisation in the dryerrestored itself. As the polymer is very sticky, due to the low sealingtemperature, continuous fluidisation is essential to prevent lumpformation which can easily plug outlet values and lines. When thishappens, a shutdown is inevitable.

1. A process for producing a copolymer of propylene with ethylene and/ora C₄ to C₈ α-olefin comprising contacting propylene and said olefinunder polymerisation conditions with an olefin polymerisation catalystwhich includes a carrier, the carrier particles included in saidcatalyst being substantially free of particles of said product having aminimum dimension greater than 55 μm.
 2. A process according to claim 1wherein said carrier is produced by spray-crystallisation of a melt of acarrier precursor to yield a spray-crystallised product containingparticles up to at least 500 μm in size.
 3. A process according to claim1 comprising contacting propylene, said olefin, and ethylene with saidcatalyst, and wherein said copolymer is a propylene/ethylene/C₄-C₈α-olefin terpolymer.
 4. A process according to claim 2 wherein saidspray-crystallised product is, after its formation, separated into aplurality of fractions, including one having a maximum particledimension of 55 μm, by screening.
 5. A process according to claim 1wherein said carrier is magnesium dichloride.
 6. A process according toclaim 2 wherein said melt is represented by the formula:MgCl₂.nROH.mED in which R is C₁-C₆ alkyl, ED is an electron donor, m=0-1and n=1-6.
 7. A process according to claim 6 wherein ED is an aromaticcarboxylic acid ester.
 8. A process according to claim 7 wherein ED is adialkyl phthalate or maleate.
 9. A process according to claim 1 whereinproduction of the copolymer or terpolymer is commenced by contactingpropylene alone with said catalyst.
 10. A process according to claim 3wherein production of said terpolymer is commenced by contactingpropylene and either ethylene or said α-olefin with said catalyst.
 11. Aprocess according to any claim 1 wherein said contacting is performed inat least one slurry reactor.
 12. A process according to claim 1 whereinsaid contacting is performed in at least one gas phase reactor.
 13. Aprocess according to claim 1 wherein said contacting is performed in atleast one slurry reactor and at least one gas phase reactor.
 14. Aprocess according to claim 13 wherein the gas phase reactor or reactorsare downstream of the slurry reactor or reactors.
 15. A processaccording to claim 14 wherein the slurry and gas phase reactors arearranged in cascade.
 16. A process according to claim 3 wherein theterpolymer comprises 0.1 to 3 wt % of units derived from ethylene and 1to 15 wt % of units derived from C₄-C₈ α-olefin.
 17. A process accordingto claim 1 wherein the α-olefin is butene.
 18. A process according toclaim 1 wherein the carrier of said polymerisation catalyst has anaverage particle size of 15 to 45 μm.
 19. A process according to claim 1wherein the olefin polymerisation catalyst system comprises astereospecific Ziegler-Natta catalyst comprising as essential componentsTi, Cl and Mg.
 20. A process according to claim 19 wherein saidcatalyst-system comprises an external electron donor and a cocatalyst.21. A process according to claim 20 wherein said cocatalyst is anorganoaluminium compound and said electron donor is a silane-baseddonor.
 22. A process according to claim 1 wherein the olefinpolymerisation catalyst is a single site catalyst comprising a singlesite/activator reaction product impregnated in the pores of saidcarrier.
 23. A process according to claim 1 utilizing spray-crystallisedMgCl₂ having an average particle size below 55 μm as carrier for apolymerisation catalyst in the production of propylene copolymers orterpolymers.